It is now clear why the ocean near India has a dent

The Earth is a sphere and the force of gravity is the same at every point on its surface. This statement is approximately correct, but strictly speaking it is wrong. In fact, the Earth is slightly flattened at the poles and slightly bulged at the equator – a result of the Earth’s rotation.

What makes the actual, detailed shape of the Earth even more complex is that gravity does not have the same force everywhere. The differences are so small that you would never notice them yourself, but they can be measured.

These differences become tangible, for example, in the world’s oceans – because here sea level is actually directly influenced by gravity. Where the Earth’s gravity is greater than in surrounding areas, the water is drawn in more strongly and the sea bulges slightly in such places. Where gravity is lower, there is a corresponding dent in sea level.

The most pronounced of these gravity anomalies are in the Indian Ocean and are called “Indian Ocean Geoid Layer” (IOGL). Here the sea surface is actually about 100 meters lower than in the surrounding areas. Because the IOGL is twice the size of the European landmass and the differences in gravity gradually decrease towards the edge, this enormous difference in height is not noticeable locally.

Nevertheless, this gravity hole is something special and has puzzled science for years. Now researchers have found a possible explanation for this phenomenon:

For their study ‘How the Indian Ocean Geoid Low was Formed’, which was published in May this year by Debanjan Pal and Attreyee Ghosh in the journal ‘Geophysical Research Letters’, the researchers used 19 computer models that show the movement of the Earth’s mantle and tectonic plates simulated 140 million years ago.

The models that best replicated the IOGL all had one thing in common: columns of hot, low-density magma rising from Earth’s interior, displacing higher-density material. As a result, part of the Earth’s mantle in this region now has significantly less mass. Therefore, the force of attraction at this point is correspondingly lower.

Another geological phenomenon known as the “African Blob” is believed to be partly to blame: a bubble of dense material lying deep in the Earth’s crust beneath the African continent.

The researchers now suspect that the following effect could have occurred as part of gigantic tectonic shifts on several continental plates: About 130 million years ago, the Indian plate broke away from the then supercontinent Gondwana and migrated towards the Eurasian plate. It is said that the Indian plate pushed itself over the plate of the ancient ocean Tethys along the way, pushing it down.

Splinters from the fractured Thetys plate now migrated deeper and deeper into the Earth’s crust near what is now East Africa. About 20 million years ago they would have displaced some of the magma trapped in the ‘African Blob’. This formed a magma column towards the Indian Ocean, which is ultimately responsible for the IOGL.

So far this is all just a theory presented in simulations. Now the researchers must prove the existence of the magma column. This will be achieved using earthquake data collected around the IOGL.